1. Field of the Invention
The present invention relates to an illumination system for use as part of a film video player. More particularly, the invention relates to flash tube simmer circuitry for an electronic strobe light of the video player.
2. Description Relative To The Background Art
U.S. Pat. No. 4,495,516, which is assigned to the assignee of the present invention, discloses a film video player for displaying on a video monitor color images recorded on color negative photographic film. The film video player includes a pulsed light source for illuminating a CCD image sensor via a pulse of high-intensity light projected onto the photographic negative at the vertical retrace rate of a standard television signal. Photosignals are read out in the dark to avoid smearing the image signal produced by the CCD. The pulsed light source comprises a quartz halogen projection lamp, the light output of which is chopped by a spinning shutter disc. Although the light source produces very adequate illumination with excellent pulse-to-pulse uniformity and constant color temperature, the chopped light source is mechanically complex and consumes a considerable amount of energy, requiring substantial cooling.
U.S. Pat. No. 4,495,516 further discloses that the pulsed light source may comprise an electronic strobe light such as a Xenon flash. A film video player with an electronic strobe flash light provides several advantages over a film video player having a quartz halogen projection lamp with a mechanical shutter. In particular, the strobe light provides reduced mechanical complexity and lower power consumption, resulting in lower heat generation and consequently reduced cooling requirements, and better color temperature stability over the life of the light source.
A film video player having a conventional electronic strobe light as the pulsed light source, produces a television signal having an annoying flicker. A measure of the flicker produced from a number of flash tubes powered with a conventional strobe light circuit indicates that the average flash-to-flash variation in light output is on the order of one stop (0.3 log E).
The aforementioned copending U.S. patent application U.S. Ser. No. 596,868, filed Apr. 4, 1984, discloses and claims a film video player having a high-voltage power supply which produces a substantially constant flash-firing current for each flash-producing operation. To achieve that result, the electronic strobe light includes a conventional flash trigger circuit, operable at the video field rate, for producing a high voltage trigger electrode pulse, which ionizes at least a portion of the gas in the flash tube. A high-voltage power supply, in response to each triggering operation of the flash tube, supplies a relatively short high-voltage pulse greater than the minimum ignition voltage of the flash tube, to ignite a flash tube arc. A regulated low-voltage power supply, in response to the high-voltage supply, then applies a voltage to the flash tube that is less than the minimum ignition voltage, but which is adequate to produce a pulse of light if an arc of the flash tube is already ignited. By producing the bulk of the light pulse with a voltage which is lower than the minimum ignition voltage of the flash tube, the amount of power that is consumed to fire the strobe light is significantly reduced.
Although the circuitry of the above mentioned patent application has been found to work well for its intended purpose, it is noted that the high voltage trigger electrode pulse produces a level of electromagnetic interference that has the potential for degrading the performance of the film video player. A further potential problem is that the flash tube is subjected to a thermal shock each time its arc is restarted during the triggering operation. It is believed that repeatedly subjecting the flash tube to a thermal shock at the video field rate (60 Hz) may limit the operational life of the flash tube. Additionally, it has been found that repeated triggering of the flash tube produces an audible signal at the 60 Hz video field rate which contributes to the overall noise related to the operation of the video player.
It is known in the electronic flash art to provide simmer circuitry which functions, after a flash tube is fired initially, to draw only a limited amount of current through the flash tube, to maintain an arc. Thus, when subsequent flashes are required, it is not necessary to apply a trigger electrode pulse. In an electronic strobe light for a film video player, simmer circuitry would serve advantageously to limit electromagnetic interference, possibly extend the useful life of the flash tube, and eliminate audible signals at the flash firing frequency.
For a film video player having a solid state image sensor, the simmer circuitry must operate to limit the current through the flash tube so that, at most, only a very slight amount of light is produced, to avoid during read out the smearing of the image signal produced by the image sensor. The simmer current, on the other hand, must be adequate to maintain an arc. As these two requirements conflict, severe limits are placed on a permissible level of simmer current.
With regard to a film video player including an electronic strobe light having simmer circuitry, it has been found through laboratory testing, that the arc of the flash tube extinguishes on a random basis. It is not known precisely why this occurs. Because of the random nature of the problem, it is believed to be caused by a variety of probalistic factors, which, under certain operating conditions, combine in a way that causes the extinguishing of the arc. Thus, it has not been possible to reliably provide an appropriate simmer current, and thereby reliably provide repeated flash pulses without a continuous flash tube triggering operation at the flash-firing rate.